A wrench is a tool for applying torque to fasteners, such as nuts and bolts, for the purpose of tightening or slackening the fastener. In general, wrenches are designed to be used on fasteners that have a polygonal drive receiving portion comprising a number of generally flat drive receiving surfaces. The head portion of an open ended wrench comprises an open-sided fastener receiving aperture having a pair of opposed fastener engaging surfaces that are spaced apart by a distance corresponding to the size of fastener they are designed to be used on. Ring wrenches have a generally circular head portion that defines a closed fastener receiving aperture. The internal surface defining the aperture is shaped to engage the type and size of fastener it is designed to be used on.
Particularly in cases in which high levels of torque need to be applied to a fastener, a ring wrench is the better tool. An open ended wrench only has two torque applying surfaces and, particularly when high levels of torque are being applied, they can easily slip off of the fastener. This usually results in damage to the fastener. Specifically, the corners formed where adjacent drive receiving surfaces meet are rounded off making it more difficult for the wrench to successfully apply high levels of torque to the fastener. Ultimately, the damage can effectively round off the fastener drive receiving surfaces to the extent a conventional wrench can no longer successfully operate the fastener. This is a particular problem where the fastener is in place and the user of the wrench is trying to loosen it.
Ring wrenches have the same number of fastener engaging surfaces as the fastener has drive receiving surfaces. With ring wrenches, because the fastener receiving aperture is closed there is no danger of the tool slipping off of the fastener and because the applied torque is distributed more evenly around the fastener, it is less likely high levels of applied torque will result in the corners of the fastener being rounded off. Unfortunately, there are many applications for which a ring wrench cannot be used. For example, where a nut is located on a pipe, it is not possible to fit a ring wrench over the pipe and so an open ended wrench has to be used.
The nut that secures a brake pipe to brake unit on an automobile is one example of a fastener that has to be worked on with an open ended wrench. Because of their position on the vehicle, these nuts are exposed to all of the adverse weather and other environmental conditions to which a vehicle is exposed. They are also relatively small in size and so in the circumferential direction of the nut, the drive receiving surfaces are relatively short. The result is that relatively high levels of torque are needed to release them. Often when such levels of torque are applied, the wrench head portion slips off rounding off the corners of the nut. This problem is often exacerbated by the location of the nut, which is typically such that only one or two pairs of drive receiving surfaces are accessible. Once the corners of those drive receiving surfaces have been wrung off, it is not usually possible to loosen the nut with a conventional open ended wrench.